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Gallois, R. W. 2001. Geoscience in south-west England, Vol. 10, 243-246 1 FIELD EXCURSION TO EXAMINE THE GEOLOGY AND COASTAL LANDFORMS OF THE CHARMOUTH TO LYME REGIS AREA, 3RD JANUARY, 2001. R. W. GALLOIS Gallois R. W. Field excursion to examine the geology and coastal landforms of the Charmouth to Lyme Regis area, 3rd January, 2001. Geoscience in south-west England, 10, 243-246. INTRODUCTION The aim of the field excursion was to demonstrate the more important geological features that have controlled the development of the landforms in the coastal zone between Charmouth and Lyme Regis, and to illustrate some of the results of recent geological studies that have been designed to reach an understanding of the processes involved. The visit was, in part, complementary to the description of the work carried out to try to provide the data required to design remedial works that will save Lyme Regis from the twin threats of landslip and marine erosion (Gallois and Davis, 2001). When the fieldtrip was first planned in Autumn 2000, the intention was to traverse the beach from Charmouth to Seven Rock Point, west of Lyme Regis, to examine the stratigraphy and structure of the Jurassic rocks exposed in the cliffs, foreshore and Black Ven, and the landslips and marine-erosion features adjacent to Lyme Regis. A number of unusual events combined immediately prior to the visit to make this impracticable. First, the unusually high rainfall in the early winter had reactivated many of the landslips along the coast. A major landslip at Cain's Folly [SY 376 928] one week before the visit led the police to close the beach and coastal path for several miles eastwards from Charmouth, and to advise extreme caution on the beaches and landslipped areas west of Charmouth. Second, south-westerly gales in December had transported much of the beach shingle offshore with the result that beach levels were up to 3 m lower than normal, albeit with better geological exposure in the intertidal zone. Third, strong winds on the day before the visit had produced a heavy swell that carried the sea to the foot of cliffs that would

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Gallois, R. W. 2001. Geoscience in south-west England, Vol. 10, 243-246

1

FIELD EXCURSION TO EXAMINE THE GEOLOGY AND COASTAL

LANDFORMS OF THE CHARMOUTH TO LYME REGIS AREA, 3RD

JANUARY, 2001.

R. W. GALLOIS

Gallois R. W. Field excursion to examine the geology and coastal landforms of the

Charmouth to Lyme Regis area, 3rd January, 2001. Geoscience in south-west England,

10, 243-246.

INTRODUCTION

The aim of the field excursion was to demonstrate the more important geological

features that have controlled the development of the landforms in the coastal zone

between Charmouth and Lyme Regis, and to illustrate some of the results of recent

geological studies that have been designed to reach an understanding of the processes

involved. The visit was, in part, complementary to the description of the work carried out

to try to provide the data required to design remedial works that will save Lyme Regis

from the twin threats of landslip and marine erosion (Gallois and Davis, 2001).

When the fieldtrip was first planned in Autumn 2000, the intention was to traverse the

beach from Charmouth to Seven Rock Point, west of Lyme Regis, to examine the

stratigraphy and structure of the Jurassic rocks exposed in the cliffs, foreshore and Black

Ven, and the landslips and marine-erosion features adjacent to Lyme Regis. A number of

unusual events combined immediately prior to the visit to make this impracticable. First,

the unusually high rainfall in the early winter had reactivated many of the landslips along

the coast. A major landslip at Cain's Folly [SY 376 928] one week before the visit led the

police to close the beach and coastal path for several miles eastwards from Charmouth,

and to advise extreme caution on the beaches and landslipped areas west of Charmouth.

Second, south-westerly gales in December had transported much of the beach shingle

offshore with the result that beach levels were up to 3 m lower than normal, albeit with

better geological exposure in the intertidal zone. Third, strong winds on the day before

the visit had produced a heavy swell that carried the sea to the foot of cliffs that would

Gallois, R. W. 2001. Geoscience in south-west England, Vol. 10, 243-246

2

usually be accessible. The field party therefore spent the morning at Charmouth and then

travelled by car to Lyme Regis for lunch and the afternoon.

CHARMOUTH AND BLACK VEN

The group followed the coastal path from Sea Lane car park [SY 365 931] at

Charmouth, in places skirting around the edge of recent landslip activity, to a National

Trust viewpoint [SY 357 934] on the eastern edge of the Tertiary plateau, adjacent to the

Roman Charmouth-Lyme Regis road. This locality provides splendid views eastwards

along the Dorset coastline and northwards inland to the Vale of Marshwood and beyond.

The westerly overstep of the Gault and Upper Greensand across the easterly dipping

Lower and Middle Jurassic rocks, the Tertiary planation surface, and extensive active

landslipping are clearly displayed in the cliffs below Stonebarrow Hill and Golden Cap.

Inland, the topographical expression of the same geology can be seen in the dissected

Tertiary plateau, the Cretaceous escarpments and the undulose landslipped hillsides that

run northwards from the coast.

To the west, the same viewpoint overlooks the great amphitheatre of the Black Ven-

Spittles landslip. The subsidiary failure mechanisms within the landslip were well

displayed at the time of the visit. A steady stream of collapsed masses of weathered sand

and chert from the Upper Greensand Formation (Whitecliff Chert Member) fell from the

backface of the landslip, and intermittent streams of sand from the lower part of the

Upper Greensand (Foxmould Member) flowed over the terrace-like escarpments that are

capped by thin (mostly ‹ 0.5 m thick) limestone beds within the Shales with Beef, Black

Ven Marl and Belemnite Marl members of the Jurassic Charmouth Mudstone Formation

(Figure 1). The principal failure mechanism, mass movement along shear surfaces in the

Gault Formation beneath the Upper Greensand, has rarely been observed because of the

almost permanent complete cover of collapsed Upper Greensand debris.

The Black Ven-Spittles landslip has been described as the largest active coastal landslip

in Europe (Brunsden, 1969). The area that is currently active consists of an upper bench

of about 0.2 km² of collapsed Upper Greensand that overlies a thin (1 to 2 m thick) bed of

Gault. Below this and extending to the sea, about 0.4 km² of mudflows and collapse-

debris derived from the Charmouth Mudstone and Upper Greensand rest on a series of

Gallois, R. W. 2001. Geoscience in south-west England, Vol. 10, 243-246

3

benches between steep faces of Charmouth Mudstone (e.g. Conway, 1974). Both parts of

the currently active area are within a larger landslip complex that was probably formed in

the late Pleistocene in a periglacial climate when it was subjected to successive periods of

freezing and thawing of a deep permafrost layer. The Holocene rise in sea level has

caused active erosion of the toe of the 'fossil' landslip, which has become progressively

reactivated as erosion has proceeded. There is historical evidence to show that the rate of

reactivation has increased in the past 200 years, and it has been suggested that this may in

part be the result of increased marine erosion due to sea defence works and quarrying on

the foreshore at Lyme Regis.

The party walked westwards, close to the backface of the landslip, to a point on the

golf course at Timber Hill [SY 350 934] near the western limit of the active face, taking

care to stay on the landward side of a long line of tension cracks that presaged further

retreat of the backface. Westwards from this point, the Upper Greensand in the backface

was at that time vegetated by scrub and trees that became thicker and more mature in a

westerly direction. Six weeks after the visit, on February 14th 2001, a failure occurred for

a distance of c. 200 m along the tension cracks that had been observed on the visit, and

Figure 1. Simplified geological section through the central part of the Black Ven landslip showing the relationship of the topography to thin limestone beds within the Charmouth Mudstone Formation.

Gault hidden by mounds of collapsed Upper Greensand

coastalfootpath

150

160

140

120

100

80

60

40

20

00

Blue Liasexposed incliffs and onforeshore

Black Ven Marl

Whitecliff Chertdecalcified in part

Grey Ledgecaps feature

Belemnite Stone caps feature

Lower Cement Bed caps feature

Birchi Tabular caps feature

Stellare Nodules cap feature

Upper Cement Bed caps feature

Blue Lias

Shales with Beef

Belemnite Marl

NorthSouth

Foxmould

Gault

mudflows and patches of collapsed sand and mudstone omitted for clarity Vertical exaggeration: x1.25

500 400 300 200 100 metres

metres

Gallois, R. W. 2001. Geoscience in south-west England, Vol. 10, 243-246

4

new tension cracks developed for another 250 m to the west. This was the longest single

reactivation of the Upper Greensand backface yet recorded, and it may prove to be the

most significant because it could regenerate a large area in the western part of the 'fossil'

landslip complex, adjacent to the Lyme Regis urban area (Figure 2).

Figure 1. .

Sketch diagrams of the western part of the Black Ven-Spittles 'fossil' landslip showing the reactivation between 1955 and 1996. Based on oblique air photographs; uncorrected for parallax

groynes

Church Cliffs

modern Lyme-Ch

armouth Road

modern Lyme-Ch

armouth Road

Upper Greensand Escarpment

1955

Stellare Nodules

Upper Cement Bed

Birchi Tabular

Belemnite Stone

Spittles Limestone

Grey Ledge

groynes

Church Cliffs

Cliffs/backfaces of Lias Mobile landslip debris

1996

'new' Charmouth Road

'new' Charmouth Road

Upper Greensand Escarpment

Upper Cement Bed

Lyme RegisUrban Area

Lyme RegisUrban Area

NENW

SW SE

SE

NENW

SW

('old' Charmouth Road)

('old' Charmouth Road)

Spittles Lane

Spittles Lane

Timber Hill

Timber Hill

probable inland limit of 'fossil' landslip

probable inland limit of 'fossil' landslip

cliffs of Blue Lias

cliffs of Blue Lias

western limit of activebackface at 14/2/2001

western limit of activebackface at time of visit

1000m

500m

900m

approximatearea of figure

1000m

500m

900m

approximatearea of figure

Belemnite Stone

Stellare Nodules

Birchi Tabular

Spittles Limestone

Grey Ledge

Gallois, R. W. 2001. Geoscience in south-west England, Vol. 10, 243-246

5

The traverse of the backface of the landslip also afforded good views of what remains

of the 'new' Lyme Regis to Charmouth road. This ran along the foot of the then wooded

and stable Upper Greensand backface, presumably to avoid the extra climb up to the

Tertiary plateau. The 'new' road was opened in 1825 as a replacement for the 'old' road

which had run eastwards through the lower part of the landslip from what is now Spittles

Lane [SY 344 927] (Figure 2). The 'new' road was damaged by landslipping within three

years of its opening and, after many repairs, was eventually replaced by the modern road,

which runs inland to avoid the landslip, in 1880 (Woodward and Ussher, 1911, p. 80).

The 'new' road was finally closed to traffic in 1924. All that remains of it are a short

section within the Devil's Bellows road cutting [SY 357 933] at the eastern end and about

300 m at the western end [SY 346 932], much of which consists of a tarmac switchback

cut by deep fissures. In contrast, the Roman Road was not breached until 1995 when a

particularly large section of Upper Greensand backface collapsed after an unusually wet

autumn. Being on the Tertiary plateau, it could have been moved inland at little cost

if that had been required.

LYME REGIS

After lunch at the Cobb Arms, Lyme Regis, the party walked through sheets of sea

spray to the point on the outer wall of The Cobb [SY 3402 9142] made famous by Meryl

Streep in The French Lieutenant's Woman. There, the geographical conditions that made

the site so favourable for a harbour were described with the help of seismic-reflection

profiles commissioned by West Dorset District Council. These show that the original

outer wall of what is now the harbour complex was built on a limestone ledge (the Birchi

Tabular Stone Band of Lang, 1914) that provided protection from the prevailing south-

westerly winds. On the lee side of this ledge, the soft Shales-with-Beef give rise to a

sand-floored channel that gave access to the sheltered area behind the wall at all states of

the tide.

Early drawings and accounts show that the Cobb wall was not connected to the shore

until the 18th century. A low causeway was built to link it to the land when a new wall

was constructed in 1756 (Fowles, 1991). With time, this wall and the causeway were

Gallois, R. W. 2001. Geoscience in south-west England, Vol. 10, 243-246

6

increased in height to produce the present-day wall, the landward end of which was last

raised in 1939 to provide year-round protection for an air-sea rescue facility.

Figure 2. .Changes in the intertidal area adjacent to The Cobb over the last 200 years

TheCobb

Cobb wall

River Lim

River Lim

River Lim

Broad Ledge

Church

Church

Church

HWMMT

HWMMT

HWMMT

HWMMT

LWMMT

LWMMT

LWMMT

LWMMT

LWMMT

LWMMT

LWMMT

LWMMT

LWMMT

Murdoch MacKenzie 1797

Admiralty chart 1854

Air Photograph 1996

Gravel (shingle) beach

Rock ledges, boulder strewn in part

Rock armour/Rip rap omitted for clarity

Sand beach

KEY

Degraded cliff of Shales-with-Beef

Landslip in Shales-with-beef

LWMMT

HWMMT High water mark median tides

Low water mark median tides

landscaped and built-up area

TheCobb

cross wall

Gallois, R. W. 2001. Geoscience in south-west England, Vol. 10, 243-246

7

These changes to the Cobb wall have interfered with the natural west-to-east longshore

drift of beach shingle. This has resulted in a steady accumulation of shingle to the west of

the wall and the consequential loss of shingle from the town beach (Figure 3). One of the

earliest photographs of Lyme, (c. 1850, published by Fowles, 1990, Plate 11) shows a

short length of Shales-with-Beef cliff below what is now Lister Gardens. The removal of

the protective belt of shingle from the foot of this cliff probably encouraged its collapse

in the late 19th century, with the result that landslip movements have continued in that

area to the present day.

As the tide receded, the party walked westwards along the intertidal rock platform to

the eastern end of Pinhay Bay. The alternating limestones and shales of the Blue Lias

Formation are continuously exposed in the cliffs and on the foreshore between

Devonshire Head [SY 332 914] and Seven Rock Point [SY 328 909]. The limestone

ledges contain what Arkell (1933, p. 121) described as "probably the greatest profusion of

large ammonites to be seen anywhere in the British Isles". Many of the limestone bands

were worked for cement manufacture up to the time of the First World War. The stone

was transported to a cement works [SY 335 916] at Monmouth Beach via a narrow-gauge

railway than ran along the intertidal area. Short stretches of this were observed below

Devonshire Head.

ACKNOWLEDGEMENTS

The author is grateful to Geoff Davis and Geoff Kellaway for helpful discussion, and to

Dick Edwards for continuing advice and encouragement. Published by permission of the

Director, British Geological Survey (NERC).

REFERENCES

ARKELL, W. J. 1933. The Jurassic System in Great Britain. Clarendon Press, Oxford.

BRUNSDEN, D. 1969. The moving cliffs of Black Ven. Geographical Magazine, 41,

372-390.

CONWAY, B W. 1974. The Black Ven landslip Charmouth, Dorset. Report of the Institute of

Geological Sciences, No. 74/3.

FOWLES, J. 1990. Lyme Regis Camera. Dovecote Press, Wimborne.

Gallois, R. W. 2001. Geoscience in south-west England, Vol. 10, 243-246

8

FOWLES, J. 1991. A short history of Lyme Regis. Dovecote Press, Wimborne.

GALLOIS, R. W. and DAVIS, G. M., 2001. Saving Lyme Regis from the sea: recent

geological investigations at Lyme Regis, Dorset. Geoscience in south-west

England, 10, 000-000.

LANG, W. D. 1914. The geology of the Charmouth cliffs, beach and foreshore. Proceedings

of the Geologists' Association, 25, 293-360.

WOODWARD, H. B. and USSHER, W. A. E. 1911. Geology of the Country near Sidmouth

and Lyme Regis. Memoir of the Geological Survey of Great Britain. Her Majesty’s

Stationery Office, London.